This invention relates to proximity sensors and related circuits for detecting the presence of metal mass radially surrounding the sensor.
Proximity sensors are used in manufacturing and material handling applications for detecting the presence or placement of metal workpieces. Generally proximity sensors generate a magnetic field from an inductor coil. When a metal workpiece is brought into the magnetic field emitted from the sensor, eddy current losses in the metal workpiece cause the impedance of the inductor coil to decrease. This decrease in impedance of the inductor coil is detected in monitor circuitry which signals the presence of a metal workpiece in the magnetic field.
One current use of such sensors is to determine whether weld nuts or pierce nuts have been properly affixed to holes formed in a vehicle body panel. Many nuts are inserted into body panels, such as vehicle body panels. Occasionally, one of the nuts may be missing or could be broken during installation. Missing nuts in body panels are sometimes not detected until after the defective metal plate or body panel has been assembled with several other components or incorporated into a vehicle, at which time remedial steps are costly and time consuming.
Current sensors are forward-looking; i.e., they only detect whether a metal workpiece is present in front of the sensor. Forward-looking proximity sensors are not ideally suited for some applications. As an example, these sensors do not always properly detect the presence of a nut fastened to a body panel as described above. The eddy current losses in the body panel itself decrease the impedance of the inductor coil, thus making the detection somewhat complex for forward looking sensors.